1. Field of the Invention
The present invention relates to an operation method for a fuel cell that generates electricity using hydrogen gas and oxidant gas as reaction gases, and, in particular, to an operation method for a fuel cell that has excellent safety control against hydrogen leakages.
2. Description of the Related Art
One type of fuel cell that is mounted in a fuel cell vehicle or the like is formed as a stack in which a plurality of cells, each formed by interposing between an anode and a cathode a solid polymer electrolytic membrane (formed, for example, from a solid polymer ion exchange membrane or the like) such that the solid polymer electrolytic membrane is sandwiched on both sides by the anode and cathode, are arranged in layers. This fuel cell is provided with a hydrogen pole to which hydrogen gas is supplied as fuel gas, and an air pole to which air containing oxygen is supplied as an oxidant gas. In this fuel cell hydrogen ions generated by a catalytic reaction at the anode move to the cathode by passing through the solid polymer electrolytic membrane, and subsequently cause an electrochemical reaction with oxygen at the cathode so as to generate electricity.
Because hydrogen gas is an inflammable gas it is necessary to take sufficient care that it does not leak into the atmosphere. Generally, in locations where hydrogen gas is handled, for safety control the hydrogen density is monitored and the necessary steps are taken in accordance with the size of the detected hydrogen density, and fuel cells that use hydrogen gas as fuel are not exceptional. However, because hydrogen is a highly permeating substance it is extremely difficult to seal it in completely. In particular, because the above described solid polymer electrolytic membrane type of fuel cell has a stack structure it is difficult to completely block the hydrogen from leaking out from the fuel cell. Moreover, it is difficult to prevent hydrogen leakages from joint portions such as where connections are formed in the hydrogen piping.
Therefore, conventionally, a hydrogen sensor that detects the hydrogen density is installed in the vicinity of the fuel cell. When the hydrogen sensor detects a hydrogen density above a predetermined level a warning is issued and the operation of the fuel cell is halted.
Moreover, as is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 8-31436, there is also a fuel cell that is housed inside a ventilated box. The hydrogen density inside the box is constantly monitored by installing a hydrogen sensor that detects the hydrogen density inside the box. When the hydrogen density detected by the hydrogen sensor exceeds a predetermined level a ventilation fan is operated and ventilation is forcibly performed, thereby reducing the hydrogen density inside the box.
However, in a system such as the conventional one in which a warning is issued and the system halted when a predetermined hydrogen density is detected by the hydrogen sensor there are times when the system is suddenly halted without any prior warning. In the case of a fuel cell vehicle, if the operation of the fuel cell is suddenly halted like this the problem arises that the vehicle cannot be moved to a location where repairs can be made.
Moreover, as is disclosed in Japanese Patent Application Laid-Open (JP-A) No. 8-31436, in a system in which a ventilation fan is operated and the interior of the box is forcibly ventilated when a predetermined hydrogen density is detected by the hydrogen sensor, because the amount of hydrogen leakage is not suppressed the fear arises that the amount of hydrogen leakage will subsequently continue to increase.